Millimeter-wave (MMW) wireless systems, operating from 30-300GHz, are emerging as a promising technology for meeting the exploding bandwidth requirements by enabling multi-Gbps speeds. For example, the 5G-oriented ultra-dense networks (UDN) will be most probably deployed in MMW band. A typical deployment for UDN is in highly populated areas such as hot spots, office building, or downtown area at cities where there are demands of high data rate service. At such high transmission frequency (e.g. ≥6 GHz), the path loss becomes much higher than that at low transmission frequency. In operation, the beacon signal containing the information such as synchronization information and random access configuration need to be broadcasted to a large enough coverage area by an access point (AP) such that all user equipment (UEs) served can receive it correctly.
FIG. 1 illustrates the individual broadcasting coverage areas by an AP when employing different antenna configurations.
Due to the high path loss, the broadcasting coverage by using omni or quasi-omni directional antenna is a very small coverage. As shown, the smallest circle indicates the broadcasting coverage using omni-antenna with normal modulation and coding rate. The middle circle indicates the broadcasting coverage using omni-antenna with low modulation and coding rate. Lowering modulation and coding rate may help enlarge the broadcasting coverage a bit, which however is not enough to accomplish the seamless coverage. As illustrated in FIG. 2, there still exist edge areas between APs that cannot be covered by the enlarged broadcasting coverage.
The largest circle indicates the broadcasting coverage using beamforming antennas, whereby the high gain beamforming are enabled. In this way, the broadcasting coverage is enlarged a lot. Typically, the beacon signal will be broadcasted by beacon sweeping, which means that the AP transmits a same beacon signal over a plurality of beams directed to different directions one after another. However, since all the beamforming antennas are required to broadcast the beacon signal periodically, the antenna power consumption is notable. Furthermore, the cell-edge UEs may receive different beacon signals from different APs, which cause interference in receiving these beacon signals at the UEs.